Most of the genetic material inside a cell is found in a small structure called the nucleus. Pathologists can see the nucleus by adding a stain called H&E to the tissue. Hyperchromatic is a word pathologists use to describe a nucleus that looks darker than normal when examined under the microscope.
Another word for hyperchromatic is hyperchromasia.
A nucleus can become hyperchromatic for different reasons. Non-cancerous cells often become hyperchromatic when they are injured. Pathologists sometimes describe these cells as reactive. Some cancers are also made almost entirely of hyperchromatic cells. In this situation, the hyperchromatic cells are used to support the diagnosis.